In recent years, cellular telephones have emerged as a must-have appliance among mobile professionals and consumers alike, growing in popularity every year since they were first introduced. Their widespread use for both voice and data communications has resulted from significant progress made in their portability, network services availability, and the miniaturization and declining cost of chips, memory, and other components. The public has come to accept that mobile service enhances business and personal communications and may contribute to personal security. Consequently, mobile communication is becoming increasingly popular, particularly for voice-grade telephone services, and more recently for data communication services.
As a result of the popularity of the mobile services, there is an increasing load on the network infrastructure, which places an increased burden on the network operators to manage their wireless networks and deploy more equipment and/or higher capacity equipment in a reasoned manner that anticipates demand. For these and other reasons, there is a need to monitor usage and estimate capacity, for various network resources. To understand the problem in the appropriate context, it may be helpful to briefly consider the structure and operation of a modem wireless communications network, and thus the resources that must be developed, deployed and managed.
The construction of a wireless communication system typically includes a number of base stations dispersed throughout the service region. The geographic service region may be thought of as made up of a number of individual radio coverage areas, which typically are called “cells.” Within each cell, a base station provides two-way radio communications through its RF front end, essentially for its assigned coverage cell. The users' mobile stations communicate over-the-air, via a standard air-link interface protocol, with one or more of the base stations. To increase capacity, the cell covered by each base station may be divided into a number of sectors, by using directional antennas. In the most common configuration, a cell is divided into three sectors, each of which covers a different 120-degree section of the cell.
Groups of base stations connect to base station controllers, and each base station controller connects to a mobile switching center. In some networks, the base stations connect directly to the mobile switching center. In either case, the mobile switching center in turn provides switching between the base stations, for example for communications between mobile subscriber stations, as well as switching of communications to and from the public switched telephone network and other mobile switching centers. In more advanced networks, one or more nodes of the network also provide a packet switched coupling to a wide area data network, for Internet access and/or for private Intranet services.
Any such wireless communication system utilizes allocated frequency resources. In a direct sequence spread spectrum type system, a relatively wide frequency band (e.g. 1.23 MHz wide) is shared by a large number of base and mobile stations. Systems in high-traffic areas may utilize two or more such allocated frequency bands. For services on a given band, spreading of digital streams with different codes differentiates logical channels on the network, and thus allows the stations to differentiate the various communications transported over the shared wide frequency band. The logical channels, and thus the spreading codes used to define the channels, are network resources, which become scarce as traffic increases. The frequency band(s) or carrier(s) also are limited resources. The capacity of any base station or sector is a function of the available resources and the amount of concurrent usage that those resources can support under the unique conditions applicable in the particular cell or sector. As existing resource capacity is consumed by increased usage, remaining capacity becomes increasingly scarce, and at some point, the carrier must determine how to modify or upgrade the network to provide resources for further increases in usage, particularly in high traffic areas.
An IS-95 type cellular communications system (i.e., a communications system implemented according to Telecommunications Industry Association (TIA) Interim Standard (IS) 95, or the like), for example, uses three different pseudonoise (PN) sequences: long PN codes, Walsh codes, and short PN codes. For a downlink transmission of a digital stream, e.g. a digitized voice signal, the base station scrambles the digital stream with a long PN sequence. After some further processing, e.g. puncturing and interleaving, the base station uses an assigned one of the Walsh codes for orthogonal spreading of the digital stream. The Walsh code effectively defines a traffic channel assigned to the mobile station for purposes of the current call or session.
The base station transceiver further spreads the stream with the short PN sequence, which results in a QPSK output. The short PN code is a 15-bit sequence. The same sequence is used in many cells, however, the transceiver for each sector uses a different time offset for the short PN sequence, to differentiate each sector. The short PN offsets are typically reused throughout a CDMA cellular system in the same manner as frequencies are reused throughout an analog cellular system. The PN offsets are assigned to a cluster of cells. This group of PN offsets is then reused multiple times within the cellular system. Each PN offset is not used by other nearby radiotelephones within a cluster as this would lead to interference on the channel and a reduction in signal quality. The QPSK output is upconverted to the appropriate frequency band and transmitted from the sector antenna on the appropriate carriers to mobile stations within the cell-sector. Essentially, a mobile station recognizes traffic directed to it based on a combination of the PN sequence offset (unique to the base station sector) and the Walsh code (assigned for transmission by the base station to the mobile station for the particular session or call).
The maximum theoretical traffic capacities of such a wireless system and of the sectors in the cells of such a system therefore are determined by the number of carriers (bands) and the number of orthogonal Walsh (identifier) codes, which are available in each sector of the system. In the present CDMA system, the maximum number of the Walsh codes is 64. Although some systems may use all of the Walsh codes for traffic channels, in most systems, up to nine of these codes are used for pilot, sync and paging functions, which leaves approximately 55 Walsh codes for traffic channels. In a typical sectorized network, each sector of a cell will have its own pilot channel, defined by its short PN offset and one of the Walsh codes (typically W0). The reduced number of orthogonal codes at least theoretically indicates how much load or traffic can be carried, i.e., the capacity of each cell sector.
In practice, however, the number of traffic channels (and therefore the number of remote terminals) that can be simultaneously supported by a given band used in any one sector is more limited. Despite the mathematical orthogonality between channels that are assigned different Walsh code sequences, interference within a given frequency band will still occur between those channels. This interference increases as more code channels are assigned until the level of interference adversely affects the integrity of the communications. Depending upon the circumstances, this interference can substantially limit the number of remote stations that can be serviced at one time by a single base station sector-carrier.
When a mobile station receives downstream communications from a single base station, the traffic to that station utilizes a single Walsh code. However, as a mobile station moves, the station transits from one sector to an adjacent sector and from one cell to another. The wireless network hands off the mobile station's communications as it crosses each boundary. During a “soft” handoff transition, the mobile station receives signals from two or more sectors and/or two or more base stations and combines the signals to produce the data stream for processing and output to the user. Consequently, the downstream traffic to the one mobile station requires a number of channels and associated Walsh code resources from the serving sectors or base stations. The mobile station releases duplicate resources only when the handoff is complete. As noted, there are a relatively limited number of orthogonal channels defined by the Walsh codes available in the communication system. To allow for the extra usage of code channels during handoff, it is necessary to reserve some resources, which further decreases effective capacity of each cell and sector.
In some instances, the number of spreading codes available is insufficient to handle the number of mobile stations requiring a traffic channel from a base station. If there is no code available at the start of a call, the call is blocked. If a code is not available during a handoff, the target cell can not take the handoff, and the network may drop the call. A base station that runs low of its scarce channel resources, or runs out of spreading codes for a given carrier of a sector, may be said to be “resource limited” because the number of available spreading codes has dropped below a predetermined threshold.
Hence, a major issue in the design of a wireless communication system is the efficient use of system capacity. Management of an existing network, particularly to timely anticipate increases in demand, requires an understanding of the network's capacity and the extent to which there are resources available to support increased usage.
Today, network service providers use one of two known methods to assess base station capacity. The most widespread method is to assign a fixed maximum value of usage to all sector-carriers in a network, based on theoretical calculations. Planning can then be based on the extent to which actual and/or expected usage approaches the theoretical capacity of particular cells. It is well known in the industry that in actual fact, each individual sector in a system has a different maximum capacity, depending on height above average terrain, clutter environment, geographic distribution of users, etc. Hence, this simplistic approach does not provide an accurate estimate of capacity, in many real-world situations.
The second method is to estimate the Erlang capacity of each sector carrier by estimating the number of actual users of the sector. The method entails: (1) obtaining operational measurement of transmitted power and peak number of users (note that the quantity “peak number of users” is obtainable from some infrastructure manufacturers); (2) finding the quotient to find “average power per user”; (3) finding the maximum number of users permittable, by dividing the maximum available transmit power by the average power per user; (4) converting the maximum number of users to the equivalent maximum Erlangs of traffic using an Erlang table lookup for a specified grade-of-service; and (5) dividing the maximum Erlangs of traffic by the current Erlangs of traffic to estimate “growth factor.” This method, while more accurate than the first method, has two significant disadvantages. First, the choice of Erlang queuing model has a significant impact upon the final calculated value of capacity. Second, this method does not directly account for hardware limitations of maximum RF power. The Erlang model has long been a mainstay of traditional telephone company planning. However, the Erlang model is predicated on discrete, equal, limited resources (e.g., lines, or “trunks” in telephony parlance). While transmit (or received) power in the wireless domain is a limited resource, it is not discrete, nor is it equally shared among users, particularly in a code division multiple access-type network. One of the basic principles of CDMA is that users are allocated exactly “enough” power to provide service, but no more, to minimize interference with other coded communications on the band. Therefore, all users use a different amount of power, and they cannot be treated as equal resource demands.
Hence a need exists for a technique to estimate values of capacity for resources of a wireless communication network, which is accurate and based on actual experience. Also, any such technique should avoid the use of Erlang queuing theory. Furthermore, a technique is needed, which directly accounts for the hardware limitations of maximum RF power within the base station equipment, for example, in a digital wireless network. If possible, any technique used to analyze capacity should also provide information about current usage and/or performance.